The studies proposed in this competitive renewal are designed to continue to define the structural and biologic characteristics of a lymphokine, Lymphocyte Chemoattractant Factor (LCF), described in our laboratory in 1982. Our original project was funded to design novel schemes for purification of LCF. These schemes were to be based upon our observations that this lymphokine was selectively secreted by histamine type 2 receptor bearing T cells in serum free media. The ultimate goals were to utilize purified material to study the way non-sensitized T cells are attracted into inflammatory foci and to achieve sufficient quantities of pure material for binding studies. In the course of the current grant period, several important discoveries about LCF have heightened its potential importance. Briefly summarized, LCF binds specifically to human CD4, inducing rises in intracellular levels of Ca and inositol trisphosphate. Subsequently, motile responses and upregulation of MHC Class II surface antigens and IL2 receptors occur. Despite these activation events following exposure to LCF, no IL2 secretion occurs and no DNA synthesis takes place. Thus, LCF is, by definition, a CD4+ T cell competence-type growth factor, and it is a natural ligand for CD4. CD4 may therefore participate in multiple ways in the T cell inflammatory process, now including amplification of the accumulation of CD4+ T cells and monocytes via both chemoattractant and growth factor activity. In order to understand the structure-function binding relationship between LCF and CD4, and understand this ligand's CD4-related membrane signal transduction complex, we propose, in this application, to complete cDNA and genomic cloning of LCF, characterize its specific binding site on CD4, and identify other membrane proteins associated with CD4 signalling, (including a Pertussis Toxin G protein, tyrosine kinase activity and other T cell membrane associated proteins). We believe these studies are a logical extension of the original grant, while potentially providing new insights into the function of a major differentiation antigen, CD4.